1. Field of the Invention
The present invention relates to diffraction gratings, and more specifically, it relates to grating designs using dielectric materials rather than metallic surfaces.
2. Description of Related and Prior Art
Diffraction gratings produced by mechanical ruling into metals have been available since the early 1900's. High groove density (up to 1800 lines/mm) gratings are also produced in dielectric materials (e.g., glass) by mechanical ruling. These gratings can produce high diffraction efficiency by overcoating with thin metallic films. Mechanically ruled gratings of high groove density (&gt;1200 lines/mm) are difficult to produce in sizes larger than approximately 20 cm.times.30 cm due to wear of the cutting edge and typically exhibit a high degree of scattered radiation.
The development of high coherence length lasers and photoresistive materials in the 1970's made possible a holographic technique for the production of gratings. Holographic gratings are produced by interfering two highly coherent laser beams in a photosensitive material. A standing wave pattern with a periodic, sinusoidal distribution of intensity is produced in the material. Subsequent development in photoresistive materials produces a periodic surface relief pattern related to the original interference pattern of laser light on the surface. This pattern can be etched into the substrate material by wet chemical, plasma and/or ion bombardment techniques (either sputtering or reactive ion etching) to produce an original master grating. The pattern from the master grating can be printed to produce secondary or "replica" gratings using numerous replication techniques.
Whether left in the photosensitive material, transfer etched into the substrate and/or replicated, high diffraction efficiency in reflection is achieved by coating the surface relief pattern with a thin metallic film similar to that applied to increase the efficiency of ruled gratings. FIG. 2 shows a prior art high efficiency metallic grating 20 on a substrate 22. Diffraction efficiency as high as 95% has been obtained with metallic gratings produced by both ruling and holographic techniques. Metallic gratings exhibit a broad-bandwidth determined by the shape and depth of the groove profile and the reflectivity of the metal (gold is typically used in the infrared region of the spectrum, silver or aluminum in the visible). Due to the inherently broad-band nature of the reflectivity of metals, frequency selectivity is accomplished only by the dispersion of the grating structure. High resolution and/or high discrimination is achieved by the use of several gratings in series. Metallic gratings cannot be used to simultaneously transmit broadband radiation in the 200 nm to 3000 nm range while diffracting select frequencies.
Finally, metallic diffraction gratings whether produced by mechanical ruling or holographic techniques, have an inherently low threshold for optical damage. State of the art commercial gratings exhibit a damage threshold of less than 600 millijoules/cm.sup.2 for nanosecond laser radiation in the infrared, with lower damage thresholds at shorter wavelengths. The damage threshold for metallic gratings is determined by absorption of the incident radiation, heat transfer within the metal and the finite thickness of the metal films overcoating the surface relief profile. The damage threshold is increased for metallic gratings by decreasing the absorption and is therefore dependent upon the incident polarization of the light and its wavelength. Metal gratings exhibit decreased absorption for light which has a transverse electric (TE) polarization where the incident electric field is polarized parallel to the grooves. However, high efficiency TE metallic gratings are difficult to fabricate since the grooves must be much deeper than an equivalent transverse magnetic grating.
The limitations of metallic gratings are fundamentally related to the optical, mechanical and thermal properties of the metal itself. Many of these limitations (low optical damage threshold, low degree of frequency selectivity, finite absorption) are shared by conventional metallic mirrors. To overcome these limitations for mirrors and beam splitters, multilayer dielectric coatings were introduced. The reflectivity of a multilayer dielectric coated material is determined by thin film interference effects within the multilayer structure. Mirror coatings with reflectivity ranging from as low as 0.1% to as high as 99.9% are readily available in the visible range of the spectrum and extending to the near infrared and ultraviolet. These coatings consist of a range of materials including fluorides, oxides, sulfides, etc. By proper selection of materials and design of the optical thickness of the layers, multilayer mirrors with controllable bandwidth and negligible absorption are now commonly produced. These multilayer dielectric mirrors overcome many of the limitations of metallic mirrors described earlier.
Prior and related art associated with dielectric gratings is described in by J. M. Elson, "Infrared light scattering from surfaces coated with multiple dielectric overlayers", Applied Optics, vol 16, pp. 2872-2881 (1977) and U.S. Pat. Nos. 4,281,894 (8/1981 by Guha), 4,828,356 (5/1989 by Hobrock, et al) and 5,009,484 (4/1991 by Gerritsen).
With the exception of U.S. Pat. No. 5,009,484, the prior art is concerned with the problem of forming extremely low efficiency diffraction gratings (&lt;0.1%) to serve as beam samplers for high power lasers. Dielectric overcoatings of either metal or dielectric grating structures as shown in FIGS. 3 and 4 (prior art) can be used as low efficiency gratings but do not function as high efficiency or frequency selective gratings. FIG. 3 shows a prior art low efficiency grating 30 on transparent substrate 32.
FIG. 4 shows a prior art low efficiency (&lt;0.1%) multilayer dielectric grating concept where a multilayer stack 40 is applied over a surface relief profile 42 on a substrate 44. FIG. 5 shows a prior art low efficiency (&lt;0.1%) multilayer grating where a shallow surface relief profile 50 is produced in the second layer (from the top 52) and then overcoated with one or more layers.
U.S. Pat. Nos. 4,281,894 and 4,828,356 describe methods of fabricating grating structures in either the top of the dielectric multilayer (U.S. Pat. No. 4,828,356) or near the top (U.S. Pat. No. 4,281,894) to produce very low diffraction efficiency (&lt;10.sup.-4) for beam sampling purposes. Both patents are concerned with the formation of extremely shallow grooves and the choice of Zinc Sulfide or Zinc Selenide as an optimized material. The required deep groove structure of high efficiency gratings (1-99%) relative to low efficiency sampling gratings (10.sup.-4) distinguishes their design and fabrication from the relatively simple fabrication of low efficiency sampling gratings.
The objective of U.S. Pat. No. 4,828,356 was to produce low efficiency surface relief gratings with a square groove shape into dielectric materials overcoating metal reflectors or multilayer structures deposited on a reflecting substrate. By using anisotropic ion etching techniques on holographically exposed substrates, shallow (.about.10 nm) grooves of controllable depth could be produced over very large areas (&gt;1 m.sup.2) resulting in uniform diffraction efficiency. By trimming the thickness of the top layer of the coating/grating combination, the efficiency of the grating could be tuned to the desired value. This patented method has been used to fabricate low to moderate efficiency gratings in multilayer coatings consisting of zinc sulfide (ZnS), thorium fluoride (ThF.sub.4), or zinc selenide (ZnSe) overcoating a metallic reflector.
U.S. Pat. No. 5,009,484 describes gratings formed on one side of thin glass substrates (windows). The grating structure is designed to diffract broadband visible radiation (sunlight) with moderately high efficiency. The gratings function in transmission only with the purpose of diffracting daylight for improved lighting in buildings. The gratings do not employ the use of any multilayer structures. They are based simply on deep grooves aligned at an appropriate angle for incident broadband radiation.